I’ve had more than a few inquiries regarding our sandbox. Here is an updated image, 2 years on, with a detail sketch of how it is built.
I used cedar fence posts from a local big box home store. I cut these into lengths from 24″ to 32″. I dug a 5-6″ wide trench 8′ in diameter and slid in the posts – varying the height as I went around the circle. Before filling I tamped the posts (which have a variable width) tight, and added a layer of landscape fabric.
I cannot take credit for the design – a family member did this in their yard with firewood, and he borrowed the idea form a local school.
Note that our soil is very sandy, and drains quickly. If I lived where water would collect around the posts I would likely line the bottom of the trench with crushed stone or sand.